1. Field of the Invention
The present invention relates to conversion of the frame rate of video material.
2. Description of the Related Art
In conversion of frame rate the simplest method is to drop frames if converting to a lower frame rate or repeat frames if converting a higher frame rate. This is referred to as the synchronization method. In many applications, the synchronization method has been superseded by interpolation methods as these avoid the obvious jumps in presented content where frames are dropped or repeated where images are moving. Interpolation is typically motion-compensated interpolation to overcome the problem of smearing on moving images.
The motion-compensated interpolation method works well for the majority of material but has difficulty with blurry fast moving images often found with material originated at the filmic frame rates of 24 or 25 fps. In this type of material simple vector fields are not able to map one image onto the next and as a result interpolation artefacts are often visible. It is also the case that, because of the relatively small difference between the frame rates of 24 and 25 fps, the synchronization method requires frames to be dropped or repeated relatively infrequently. One useful approach is therefor to combine the synchronization and interpolation methods, essentially using at any one time the method that produces the better result. An issue which has to be resolved is delay. The interpolation method will be expected to have a fixed delay, while it is of the nature of synchronization that the delay is variable. It is accordingly not possible to switch arbitrarily between the two methods.
Reference is directed to WO 2011/073693 for an arrangement which combines synchronization and interpolation methods, with the issue of delay being resolved through careful control of the two methods. However, it would be advantageous if synchronization and interpolation methods could be combined in a way which required no or minimal control of the individual methods and, particularly, of the interpolation method. This would enable a choice to be made of different interpolation technologies, some of which may not offer external control over timing and delay.